High temperature cobalt and nickel-based superalloys are widely used to form certain components of gas turbine engines, including combustors and turbine vanes and blades. While high-temperature superalloy components are often formed by casting, circumstances exist where superalloy components are preferably or are required to be fabricated by welding. For example, components having complex configurations, such as turbine midframes and shroud support rings, can be more readily fabricated by welding separate castings together. Welding is also widely used as a method for restoring blade tips and for repairing cracks and other surface discontinuities in superalloy components caused by thermal cycling or foreign object impact. Because the cost of components formed from high-temperature cobalt and nickel-based superalloys is relatively high, restoring/repairing these components is typically more desirable than replacing them when they become worn or damaged.
In the past, superalloy components of gas turbine engines have been welded at an elevated temperature (e.g., in excess of about 1500.degree. F. (about 815.degree. C.)) to improve welding yields. Welding is often performed in an enclosure containing a controlled atmosphere (e.g., an inert gas) using such welding techniques as tungsten inert gas (TIG) and laser welding processes. Heating is typically performed by induction or with the use of lamps, such as quartz halogen lamps. Superalloy components of gas turbine engines are typically thermally stress-relieved before welding to relax residual stresses present from engine service, and then stress-relieved after welding to relax residual stresses induced during cool down from the welding operation. Heat treatment also provides stress relief by dissolution of a portion of hardening gamma prime (.gamma.') in .gamma.'-strengthened nickel-base superalloys. Generally, the heat treatment and welding parameters will vary depending on the alloy of interest, the amount of residual stress relief and dissolution required, furnace design, component geometry and many other factors.
TIG and laser welding techniques as described above have been successfully practiced with superalloy components. With these techniques, though a general effort is made to limit heating to the area to be welded, a relatively large area of the component is often heated. As a general rule, excessively high welding temperatures must be avoided to prevent undesired recrystallization or melting of a component, while the minimum component temperature must be sufficiently high (e.g., 1500.degree. F.) to inhibit cracking during welding. At such high temperatures, the heating and cooling cycles can be lengthy, and the comfort of the operator of the welding apparatus can be an issue.